The present invention relates to an inspecting method and apparatus for a photomask pattern which is used in fabrication of a semiconductor device such as a semiconductor integrated circuit (IC) or a large scale semiconductor integrated circuit (LSI) and, more particularly, to an apparatus and method which automatically compares the difference in shear between unit patterns on a photomask.
Before describing the present invention, it will be necessary to explain what is required of and how to fabricate a photomask pattern. Photomasks and the devices produced therefrom generally have the following characteristics: (1) a plurality of photomask patterns are required to fabricate a semiconductor device, because the semiconductor device is fabricated by repeated printing and etching of the photomask patterns on a semiconductor wafer; (2) each photomask pattern includes a large number, for example, several thousand, unit patterns on each semiconductor die, each unit pattern having the same size and shape, so that mass production of semiconductor devices from a wafer as large as five inches in diameter is possible; (3) each photomask pattern is very complicated and achieves high packing density of the semiconductor circuits; and (4) each photomask must have a high accuracy on the order of one micron.
A photomask is made by coating a glass plate with an optically sensitive material called a sensitive plate. Then unit patterns are printed on the photomask by step and repeat exposures using an optical image produced by an original unit pattern. In the step and repeat process, the photomask is mounted on a stage and the fabrication can be accomplished by shifting the stage in a step and repeat movement while using a fixed optical system for producing the exposure.
As mentioned above, the photomask is very important in the fabrication of a semiconductor device and, as a result, must be inspected very carefully. Attention must be particularly directed to irregularities that can occur, such as irregular shifting of the stage in the step and repeat movement or rotation of the optical system, both of which can occur during mask production. Such an irregularity will cause incorrect printing on the wafer, therefore, after fabricating, the photomask pattern must be inspected, especially to determine whether the unit patterns are printed in correct positions so that wasted time can be avoided during semiconductor device production.
In the prior art inspection method, a vernier pattern is provided on each edge of each unit pattern (generally a unit pattern has four sides) and the vernier patterns of neighboring unit patterns should be adjacent to each other. A visual inspection by an inspector using a microscope determines whether vernier patterns in adjacent unit patterns are within a specified tolerance of each other. However, the large number of unit patterns on the photomask require a great deal of time to visually observe all the vernier patterns on the photomask. For example, if the total number of the unit patterns on the photomask is 5,000 and it takes 20 seconds to check the vernier patterns for adjacent unit patterns, it will take well in excess of 15 hours to inspect one photomask, thereby making such a complete inspection practically impossible in actual practice. Therefore, random inspection has been performed on selected points on the photomask pattern to reduce the inspecting time to several minutes. However, as the packing density of the semiconductor device increases, a higher degree of accuracy, such as tolerances of less than one micron, is required. The random sampling inspection method mentioned above is inadequate for such small tolerance photomasks and inspection of all the unit patterns becomes necessary.
FIG. 1 is an example of an original unit pattern 1 having prior art vernier patterns 3. In the middle of unit pattern 1 is a die pattern 2, and four vernier patterns 3a, 3b, 3c and 3d are provided near the edges of the four sides of the unit pattern. The original unit pattern 1 is printed a plurality of times on a sensitive plate as represented by the photomask pattern 4 in FIG. 2. As mentioned above, the printing is made using a stage shifted in a step and repeat movement process. The movement process is very carefully controlled, however, a slight error in movement cannot be avoided in the mechanical system and the pitch between neighboring unit patterns in the latitudinal or longitudinal direction is not always equal. Of course, the pitch can have some allowable error depending upon the feature tolerance of the photomask pattern. The vernier patterns are provided to allow the inspection to measure whether the arrangement of the unit patterns is within the allowed tolerance. FIG. 3(a) illustrates an enlarged view of the vernier patterns in FIG. 2. A combination of patterns 3a and 3b in each unit pattern (2A-2F) forms a latitudinal vernier pattern. FIG. 3(b) is a further enlargement of adjacent prior art vernier patterns. In these patterns, the distance between marks is different in the adjacent vernier patterns 3c and 3d, that is, the distance "a" is greater than the distance "b". The point where the two scales match indicates the relative shear between the unit patterns. If the readings of the respective vernier patterns are within a designated allowance, then the longitudinal arrangement of the unit patterns is good. The same inspection can be made for the latitudinal arrangement by observing latitudinal vernier patterns formed by patterns 3a and 3b in each pair of adjacent unit patterns. Such a visual inspection wastes a lot of labor and time, and allows the photomask pattern to be damaged or allows dust to stick to the photomask pattern, all problems in the prior art inspecting method.